CP - Transport Across Cell Membranes Flashcards
What is non-mediated transport?
Transport that doesn’t directly use a transport protein
What is mediated transport?
Move materials with the help of transport proteins
What is passive transport?
Moves substances down their conc. or electrochemical gradients with only their kinetic energy
What is active transport?
Uses energy to drive substances against their conc. or electrochemical gradients
What is vesicular transport?
Moves materials across membranes in small vesicles either by exocytosis or endocytosis
What type of diffusion is non-mediated transport and why?
Passive diffusion, because there is no transport protein to make molecules go against diffusion laws so conc. gradient is higher outside the cell than inside
What is non-mediated diffusion important for?
Absorption of nutrients and excretion of wastes
What types of molecules is transported by non-mediated transport?
Non-polar, hydrophobic molecules, e.g. O2, CO2, N2, fatty acids, steriods, small alcohols, NH3, fat-soluble vitamins (A, D, E, K)
What are ion channels?
Water filled pores that span the hydrophobic core of the lipid bilayer and allow passage of ions (and small molecules) across the cell membrane
What do ions not bind to?
Channel pore, so transport is very rapid
What is the water filled pore lined by?
Hydrophilic amino acids (i think it might be hydrophobic? - perla)
What determines the ionic selectivity of ion channels?
Specific amino acids lining the pore
By being selective to particular ion, the channel can?
Harness the energy stored in the different ion gradients
What can discriminate ions based on their size and the amount of water they have around them?
Shape of selectivity filter
What do channels contain to control the opening and closing of the pore?
Gates
What can control the (gate) channel from opening and closing?
Different stimulis, e.g. Voltage, Ligand binding, Cell volume (stretch), pH and Phosphorylation
How can we measure ion channel function/the current flowing through an individual channel?
Patch clamp technique
What do current fluctuations represent?
Opening and closing of single ion channels and the changes in channel structure that are associated with channel gating
The diffusion of ions through a channel generates a measurable current across the membrane of how much?
10^-12 amps (dont need to know) - perla
What types of transport can carrier mediated transport be?
Passive (faciliated) or active
What is carrier mediated transport?
The substrate to be transported directly interacts with the transporter protein
Why are transport rates slower for carrier mediated transport than those obtained for channels?
Transporter undergoes a conformational change
Carrier transport proteins have similar properties to ___
enzymes
What do transport proteins do? (+ do they catalyze chemical reactions?)
They mediated transport across the cell membrane at a faster rate than normal (they don’t catalyze chemical reactions)
What do carriers exhibit?
Specificity; Inhibition; Competition; Saturation (transport max.)
What do transporters display?
Enzyme kinetics
When does glucose transport occur until?
All binding sites are saturated
What is the term to describe passive mediated transport?
Facilitated diffusion
What happens in facilitated diffusion of glucose (3 steps)?
- Glucose binds to a transport protein (GLUT)
- Transport protein changes shape and glucose moves across cell membrane (diffusing down its conc. gradient)
- Kinase enzyme reduces glucose conc. inside the cell by converting glucose into glucose-6-phosphate. This conversion of glucose maintains the conc. gradient for continued glucose absorption
What is primary active transport?
Energy is directly derived from hydrolysis of ATP
How much % of a typical cell’s energy (ATP) does it use on primary active transport?
30% dont need to know this - perla
What is secondary active transport?
Energy stored in an ionic conc. gradient is used to drive the active transport of a molecule against its gradient
OR
Energy released by a substance diffusing down its conc. gradient is used to actively transport another substance against its conc. gradient
What type of active transporter is the Na/K ATPase pump?
Primary active transporter
What does the Na/K ATPase pump maintain?
Low conc. of Na+ and high conc. of K+ in the cytosol
How does the Na/K ATPase pump work (4 steps)?
- Na+ binds to the binding pocket
- The binding causes ATP to convert to ADP and a phosphate group attaches to the pump, causing it to change its shape. Na+ is removed.
- K+ binds and phosphate group detaches
- The release of phosphate group reverts pump back to its original form and K+ is pumped into the cell
How many Na+ ions are removed and how many K+ ions are pumped into the cell?
3 Na+ ions and 2 K+ ions (don’t need to know)
What does the Na/K ATPase pump generate?
A net current and is electrogenic
What is the difference in ion conc. important for?
- Maintain resting membrane potential
- Electrical excitability
- Contraction of muscle
- Mainteinance of steady state volume
- Uptake of nutrients via secondary active transport
- Mainteinance of intracellular pH by secondary active transporters
What is the pump-leak hypothesis?
The pump works continuously due to the continual leakage of Na+ ions into the cell and K+ ions out of the cell (down their respective gradients)
What are other examples of primary active transport?
Ca2+/K+ ATPase (in muscle cells) and H+/K+ ATPase (in stomach)
What is secondary active transport?
Uses the energy stored in an ion gradient created by primary active transporters to move other substances against their own conc. gradient (indirectly use the energy obtained by hydrolysis of ATP)
Cells have many secondary active transporters which are powered by what gradient?
Na+ gradient, initially established by the Na/K pump
What occurs in Na+ antiporters or exchangers?
When Ca2+ or H+ binds to the protein, Na+ ions rush inward (down its electrochemical gradient) and Ca2+ or H+ ions pushed out
What occurs in Na+ symporters or cotransporters?
When glucose or amino acids binds to the protein, it opens the channel, so that Na+ ions diffuse down its electrochemical gradient, together with glucose or amino acids rushing inwards (against their conc. gradients)
